This invention pertains generally to tracking radars, and particularly to tracking radars using pulse Doppler techniques.
It is well known in the art that a so-called pulse Doppler radar may be used to advantage to determine, along with range and direction, the Doppler velocity of an airborne target (hereinafter referred to simply as a target) relative to such a radar. It is equally well known that, unless care is taken, any pulse Doppler radar may suffer from the effects of ambiguities in measurement of either, or both, range and Doppler velocity.
Although it is known to change the pulse repetition frequency (PRF) of any pulse radar, including a pulse Doppler radar, to eliminate any ambiguity in the measurement of Doppler velocity (and concomitant "blind speeds") and it is known to change the PRF of any pulse radar, again including a pulse Doppler radar, to eliminate any ambiguity in the measurement of range (and concomitant "blind ranges"), there has heretofore been no way, without suffering a large detection loss and complex processing to detect multiple targets simultaneously havinq low and high Doppler velocities and to resolve the Doppler ambiguity problem, in which both parameters could be simultaneously adjusted to optimize operation of a pulse Doppler in an earth satellite (referred to hereinafter as a satellite) moving in orbit.
When a pulse Doppler radar is used in a satellite in orbit some hundreds of miles above the surface of the earth to search for a relatively small target, provision must be made to reduce the effect of clutter received either in the main lobe or a side lobe of the pulse Doppler radar. At the same time, care must be taken to avoid loss of echo signals from any desired target even when the Doppler shift frequency of the echo signals from the target approachs or coincides with the Doppler shift frequency of any clutter signals. In addition, it is highly desirable that the minimum Doppler velocity that may be measured be kept as low as possible so that detection of any target is independent of the actual course of a target relative to the satellite. In addition to the foregoing, it is a further problem to reduce the size and power requirements of any equipment in a satellite to a minimum.
It has been suggested that bursts of interrogating pulses at different "medium" pulse repetition frequencies (meaning pulse repetition frequencies at which both Doppler blind speeds and range eclipsing of targets of interest are normally experienced) may be used to produce echo (meaning from a target) and clutter signals that may be processed to achieve a satisfactory probability of detection of targets. The contemplated processing includes a type of processing referred to herein as the Displaced Phase Center Antenna (DPCA) technique. In the past such a technique basically has involved: (a) making the antenna appear to received signals to be stationary even though the antenna may be mounted on a satellite moving in orbit; (b) using any known cancellation arrangement selectively to attenuate clutter signals while significantly decreasing the echo signal amplitude for many targets when such reduction cannot be afforded; and (c) utilizing the complex processing techniques to extract a true range measurement from two or more individually ambiguous measurements.
Finally, it has been recognized that other shortcomings of known pulse Doppler radars for use in satellites exist. For example, losses resulting from gating out main lobe clutter transients in the received signals, i.e., "window" losses, can be significant in known pulse Doppler radars for satellites.